The present invention relates to a battery system with a battery which is designed to supply a high-voltage network with electrical energy and a measuring device for measuring at least one insulation resistance of the battery, and a corresponding method for measuring at least one insulation resistance of a battery which is designed to supply a high-voltage network with electrical energy. The invention also relates to a vehicle having such a battery system.
FIG. 1 shows a battery system 10 known from the prior art, with a battery 20 configured as a high-voltage battery or traction battery, and a measuring device 30 for measuring at least one insulation resistance of the battery 20, which is present inside or outside the battery 20. The battery 20 comprises a plurality of series-connected battery cells. In the interests of simpler representation, said plurality of battery cells are not identified by reference numbers. Herein, a first insulation resistance 26 is the insulation resistance of the battery 20 which is present between a positive high-voltage terminal 21 of the battery 20 and a point 25 having a potential of a housing of the battery 20, hereinafter designated as the housing potential 25. Moreover, a second insulation resistance 27 is the insulation resistance of the battery 20 which is present between a negative high-voltage terminal 22 of the battery 20 and the housing potential 25. The positive high-voltage terminal 21 of the battery 20 is hereinafter designated as the first high-voltage terminal 21 of the battery 20. The negative high-voltage terminal 22 of the battery 20 is hereinafter designated as the second high-voltage terminal 21 of the battery 20. Moreover, a third insulation resistance 28 is the insulation resistance of the battery 20 which is present between a connection point, by means of which at least two battery cells in the battery 20 are interconnected, and the housing potential 25.
The measuring device 30, arranged in a battery control device (not represented) of the battery system 10, comprises a first measuring path 40 and a second measuring path 50, each of which comprises a high-resistance voltage divider 41, 51 and a relay 45, 55, and are connectable between an assigned high-voltage terminal 21, 22 of the two high-voltage terminals 21, 22 and the housing potential 25. Herein, the first high-voltage terminal 21 is assigned to the first measuring path 40. The second high-voltage terminal 22 is, moreover, assigned to the second measuring path 50. Each voltage divider 41, 51 is comprised of a first resistor 42, 52 and a second resistor 43, 53. Herein, each first resistor 42, 52 is connected to the housing potential 25, and each second resistor 43, 53 is connectable, via the corresponding relay 45, 55, to its assigned high-voltage terminal 21, 22. In the event of an insulation fault between the battery 20 and the housing, during the alternating closing of the relays 45, 55 on the measuring paths 40, 50, a measurable current flux flows through at least one of the voltage dividers 41, 51, such that a measurable voltage also drops across the first resistor 42, 52 of the at least one voltage divider 41, 51. Each voltage drop across the first resistor 42, 52 of each voltage divider 41, 51 is measured via a respective measuring input 49, 59 of an evaluation and control unit 60 of the measuring device 30 which is assigned to the corresponding measuring path 40, 50. The measuring input 49 which is assigned to the first measuring path 40 is connected, via a further resistor 46, to that terminal of the first resistor 42 on the voltage divider 41 of the first measuring path 40 which is not connected to the housing potential 25. For the measurement of the voltage drop across the first resistor 52 on the voltage divider 51 of the second measuring path 50, the measuring device 30 is equipped with an operational amplifier 56 which is connected, on the input side, to the first resistor 52 on the voltage divider 51 of the second measuring path 50 and, on the output side, to the measuring input 59 of the evaluation and control unit 60 assigned to the second measuring path 50. The evaluation and control unit 60 thus defines the respective voltage drops across the first resistors 42, 52 on the voltage dividers 41, 51. By the alternating switching-in of the measuring paths 40, 50, the voltage drop across each of the first resistors 42, 52 is determined and, in consideration of known system variables, the insulation resistances 26, 27, 28 of the battery 20 can be calculated in each case. The evaluation and control unit 60 is moreover designed to control the relays 45, 55, i.e. to open and close the latter.
The battery 20 represented in FIG. 1 is designed to a supply a high-voltage network 70 with electrical energy. To this end, the battery 20 is connectable to the high-voltage network 70 via two further relays 71, 72. Via the two further relays 71, 72, the first high-voltage terminal 21 of the battery 20 is directly connectable to a first high-voltage network terminal 73 on the high-voltage network 70, and the second high-voltage terminal 22 of the battery 20 is directly connectable to a second high-voltage network terminal 74 on the high-voltage network 70. Between the two high-voltage network terminals 73, 74 on the high-voltage network 70, a series-connected arrangement of two capacitors 75, 76 (Y-capacitors) is provided, connected in parallel with an intermediate circuit capacitor 77, which forms an intermediate circuit. A connection point, by means of which the two series-connected capacitors 75, 76 are connected, is connected to a housing potential 25. The high-voltage network 70 moreover comprises an inverter 78 and a motor 79. On its input side, the inverter 78 is connected in parallel with the intermediate circuit capacitor 77, and is designed to convert a DC voltage which can be delivered by the battery 20 into a three-phase AC voltage which is then delivered, on its output side, to the motor 79.
According to the prior art, dielectric withstand between a low-voltage network, which is galvanically separated from the high-voltage network 70, and the battery 20, or between the housing potential 25 and the battery 20, must be metrologically proven, both in-factory and during the conduct of all repairs on the battery 20. To this end, in the course of dielectric withstand testing (by a dielectric withstand test) using the battery control device (not represented) which is switched to a test mode, a test voltage is applied between the housing potential 25 and each high-voltage terminal 21, 22 of the battery 20 for a specified time. The test criterion to be used for this purpose is a measured current value. The test voltage here is delivered in the form of a DC voltage or an AC voltage.
In order to protect the electronic measuring systems of the measuring device 30 against damage during dielectric withstand testing, and to prevent any overshoot of a predefined limiting current value by the current value of any measuring current generated in course of insulation resistance measurement, the measuring paths 40, 50 must be configured with a correspondingly high resistance. As the predefined limiting current value cannot be selected to be infinitely small, each measuring path 40, 50, in a time during which dielectric withstand testing is in progress, and in a time during which no insulation resistance measurement is taking place, is galvanically separated from the battery 20, and thus also from the high-voltage network 70, wherein the relays (electromechanical switches) 44, 55 are opened.
According to the prior art, the measuring device 30, during each insulation resistance measurement, is connected via the relay 45 on the first measuring path 40 or via the relay 55 on the second measuring path 50 to the battery 20. During its service life, each relay 45, 55 undergoes from several tens of thousands up to a hundred thousand switching operations or switching cycles. The low measuring currents which arise during insulation resistance measurements, together with high switching voltages, which comprise voltage values of several hundred volts, are conducive to low-energy corona discharge during switching operations on these relays 45, 55. In combination with potential gas emissions from the relay housing material, this can lead to unwanted carbonization or film-formation on the switching contacts of the relays 45, 55. A current-related switching contact transition resistance can occur on the relays 45, 55, thereby invalidating the measurement.
From document CN 101603986 A, a high-voltage insulation resistance measuring circuit is known, comprising a high-voltage switching unit with a plurality of relays and a voltage measuring unit wherein, in a test mode, various components of the voltage measuring unit deliver a high-voltage signal via the closed relays.